FIG. 1 shows three stations operating in accordance with a power-saving scheme as specified in the IEEE standard 802.11-2012, the entire disclosure of which is incorporated by reference herein. A station that operates in accordance with such a power-saving scheme (or any other power-saving scheme described herein) is referred to herein as a “power-saving station”. In FIG. 1, three stations A, B, and C form an independent basic service set (IBSS), in an arrangement referred to as ad hoc mode. Each of the stations alternates between a doze (or, sleep) state, in which data is not transmitted or received, and an awake state in which data can be transmitted and received. Entering the doze state may also be referred to as entering a power-save mode, and waking up may also be referred to as exiting the power-save mode. Each of the stations is required to wake up just before each beacon, three of which are shown at 104-1, 104-2, and 104-3 (collectively, “beacons 104”).
After waking up for a beacon, each station remains awake for a period of time called an announcement traffic indication message (ATIM) window. During the ATIM window, each station determines whether buffered data can be sent to other stations and whether to expect to receive data buffered from other stations. Data may be sent in networking frames or networking packets. The networking frames/packets may carry data and/or control signals, such as acknowledgement signals.
For example, see the ATIM window following beacon 104-2 of FIG. 1. Station A transmits an ATIM frame 108 to station B, which is shown receiving the ATIM frame at 112. Station B acknowledges that station B will wait to receive data from station A by transmitting an acknowledgment 116, shown being received by station A at 120.
Because station A has identified an available recipient for a buffered frame, and because station B expects to receive a frame, stations A and B will remain in the awake state after the ATIM window and not return to the doze state. By contrast, station C, which does not have frames to transmit to stations A or B, and has not been notified of any queued frames at station A or station B, returns to the doze state at the end of the ATIM window.
As prearranged, after the ATIM window, station A transmits a data frame 124 to station B, which is shown receiving the data frame at 128. Station B signals a successful transmission by transmitting an acknowledgement 132 to station A, shown being received at 136. Stations A and B then remain awake for the remainder of the beacon interval—i.e., the time until the next beacon 104-3. Stations A and B, along with station C, are all awake for the ATIM window following beacon 104-3. This approach allows stations to doze for a significant portion of each beacon interval, and only remain awake when data transfer has been prearranged.